Lectins are proteins characterized by the presence of a carbohydrate binding domain. Collectins are a group of lectins comprising an oligomeric structure of subunits, each subunit having a carbohydrate binding domain. In vivo lectins are represented in a variety of number of subunits leading to populations of lectins with different molecular mass.
MBL is a protein of the collectin family and characterized by an oligomeric structure of subunits each consisting of a calcium-dependent, C-type carbohydrate-recognition domain (CRD), attached to a collagenous rod. MBL activates the complement system via associated serine proteases (MASP—mannose-binding lectin associated serine proteases), i.e. by a mechanism similar to C1q. Mannose-binding protein (MBL) is a protein to be used for substitution or replacement therapy in patients with inherited or acquired MBL-deficiency associated with functional and/or clinical symptoms.
MBL derived from human blood plasma is assembled into an oligomer of subunits, each consisting of three identical polypeptide chains. The number of subunits in an MBL molecule is varying [Lipscombe R J, et al: Distinct physicochemical characteristics of human mannose binding protein expressed by individuals of differing genotype, Immunology 85 (1995) 660–667.], but it has been suggested that the biologically active polypeptide is an oligomer consisting of more than three subunits. Plasma comprises oligomers of more than three subunits as well as denatured and structurally impaired protein forms leading to bands on for example SDS gels between the dominating MBL bands corresponding to the higher oligomers.
Recombinantly produced MBL reveals an oligomer variation similar to plasma-derived MBL [Vorup-Jensen T et al: Recombinant expression of human mannan-binding lectin, Int. Immunopharm. 1 (2001) 677–687]. However, recombinantly produced MBL usually has a higher content of low-mass forms than plasma derived MBL. Low-mass forms of MBL include for example single polypeptide chains, single subunits, and dimeric subunits.
Lectins are typically isolated by applying a composition comprising the lectins to a column being modified with a sugar to which the lectins bind. However, the efficacy of the columns varies. In PCT application WO 00/70043 a method for separating high oligomers from low mass forms is described, wherein recombinantly produced MBL is subjected to fractionation on a special column, wherein the column as such does not have any affinity for the MBL.